1. Field of Invention
The invention relates to a transformer, and, in particular, to a transformer having a high current density.
2. Related Art
A transformer is an electronic assembly, which is frequently used in various electronic apparatuses. Referring to FIG. 1, a conventional transformer 1 includes a bobbin 11, a coil set 12, a pin seat 14 and a core 15. The coil set 12 includes a primary coil and a secondary coil each formed by winding a conductive wire around a winding region 111 of the bobbin 11. The primary coil is electromagnetically coupled with the secondary coil. The pin seat 14 is connected with the bobbin 11 and includes a plurality of pins 141, which is for electrically connecting with a circuit board 17. The core 15 is composed of a first core 151 and a second core 152, which are respectively fit with two ends of the bobbin 11 and are connected together to form an electromagnetic loop.
In general, the coil set is made in the form of concentrated or folded winding. The concentrated coil set 12 is directly wound around the bobbin 11, while the folded winding structure has a primary coil and a secondary coil each wound to form a wire cake 20, as shown in FIG. 2, and then the wire cakes 20 are alternately arranged along an axial direction of the bobbin 11 and then fit with the bobbin 11. The conventional wire cake 20 is a single-layer spiral wire cake formed by spirally winding a conductive wire outward on a plane, and both ends (i.e., a beginning end 20a and a terminating end 20b) of the conductive wire are suspended outside the structure of the wire cake 20. The beginning end 20a of the conductive wire turns from inside and thus extends outside the structure of the wire cake 20 so that the beginning end 20a may be electrically connected with a pin or connected to a power source. However, under large capacity and high current requirements, the number of turns of the coil of transformer 1 is typically increased. However, the beginning end 20a of the wire has to turn outward from the inner diameter of the wire cake 20 due to the conventional winding method of the single-layer spiral wire cake. Thus, when plural wire cake structures are connected in series, a gap exists between the wire cakes due to the width of the conductive wire. The existence of the gap tends to reduce the extent of coupling between the windings.
Therefore, it is an important subject to provide a transformer having a high extent of coupling between the windings and high current density.